The present invention relates to liquid fuel burners having low emission of NOx and of dust, and also to liquid atomizers (or spray nozzles) for such burners.
The technical field of the invention is that of manufacturing industrial burners of liquid fuel (in particular fuel-oil).
Recent developments in liquid fuel burners have sought to reduce polluting emissions (un-burned solids containing carbon, CO, NOx, O2 involved in forming SO3), while maintaining the operating performance of boilers, hot gas generators, and incinerators, and in particular while maintaining the radiation from flames in the hearth, the dimensions of hearths (and thus flame dimensions), the ability to accommodate possible variations in load, and the absence of deposits of coke or scale on the tubes of the hearth, on the stabilizer of the burner, or on its refractory door.
A technique known as flue gas recirculation (FGR) consists in recirculating (or xe2x80x9crecyclingxe2x80x9d) flue gases having a low O2 content in the combustion air of burners, with the effects firstly of reducing the concentration of O2 in the oxygen carrier so as to decrease xe2x80x9ccombustiblexe2x80x9d NOx, and secondly of reducing the combustion temperature so as to reduce xe2x80x9cthermalxe2x80x9d NOx that is formed by oxidizing the nitrogen in the oxygen carrier (where such formation is highly temperature sensitive). That technique has its limits: in particular it slows down the combustion process and consequently often leads to an unacceptable increase in flame length, and also to a large increase in the emission of unburned carbon-containing solids, and also of CO and of unburned hydrocarbons.
A technique sometimes known as over firing air (OFA) ports consists in spreading out combustion as a whole by causing the burners to operate with too little air, and then adding air via ports further downstream in the flue gas circuit. That technique also has its limits: the quality of the mixing obtained between air and the combustion gas with such a method is poor, given that the air injected through the walls of the hearth cannot mix homogenously throughout the entire combustion gas stream, and as a result performance is unsatisfactory in terms of unburned solids and gases since mixing is poorly organized. It would be preferable to organize such mixing at each injection of fuel, i.e. at each burner.
Finally, a technique known as xe2x80x9creburningxe2x80x9d consists in injecting fuel, often a gaseous fuel, at the end of the hearth to reduce NOx to N, and then to add combustion air for that fuel at a subsequent stage. That technique is expensive and complicated and in any event gives a better final result when the NOx has already been kept to a minimum at each burner.
Consequently, it is still necessary, even when using the FGR technique, OFA port spreading, or reburning, to keep down emissions of NOx while maintaining emissions of unburned solids or gases and of O2 that are as low as possible at each burner. It is advantageous to keep down pollutant emission from the beginning of combustion, i.e. at the nozzle-carrying heads even if additional methods are also used for reducing emissions overall.
Studies performed on reducing NOx at burners (without significantly increasing unburned content) have shown that it is advantageous to cause flames to split up into a plurality of individual flames so as to cause NOx emission to drop by various effects as described in EP 0 435 735 (IFP-Pillard), which corresponds to U.S. Pat. No. 5,169,304.
The advantage of increasing flame separation while shortening each individual flame by staged injection of air directed exactly on each individual flame has been demonstrated, since it makes it possible to maintain flame diameters and lengths unchanged and to further improve performance in terms of unburned content and in reduction of NOx, as described in patent U.S. Pat. No. 5,562,437 (Pillard).
Those improvements do not eliminate the need to improve atomizing (or spraying) of liquid fuels by using an auxiliary atomizing fluid (saturated or superheated steam, compressed air, compressed gas).
To this end, patent FR 2 641 365 proposes a burner having a primary spray piece which defines an axial mixing chamber into which the liquid fuel is introduced axially and into which an auxiliary fluid under pressure is introduced radially or tangentially. The chamber has primary spray nozzles each having an outlet orifice from which a jet of fuel spray issues. The burner also has a secondary spray piece which surrounds the primary spray piece and co-operates therewith to define an auxiliary chamber. The secondary spray piece carries a plurality of secondary nozzles, each of which is coaxial with a primary nozzle and has a diameter greater than the outlet orifice of the primary nozzle. The burner has means for injecting an auxiliary fluid radially into the auxiliary chamber at a pressure which is sufficient to enable it to expand in the secondary nozzle around the jets issuing from the primary nozzles, and to do so at an axial speed that is greater than the speed of the peripheral layers of the sprayed fuel jets.
In addition to its primary and secondary nozzle-carrying pieces, the burner head (or end) described in that document also includes a plurality of removable pieces disposed between the nozzle-carrying pieces and the burner body. An emulsifier defines a mixing chamber for the fuel and the auxiliary fluid and has passages (for the auxiliary fluid) fitted with fins, and also has a diaphragm (or pellet) fitted with a calibrated central orifice for introducing the fuel into the mixing chamber, and a fuel guide piece (sometimes referred to as an xe2x80x9catomizerxe2x80x9d) upstream from the diaphragm. Those various pieces are generally in the form of thick disks and they are xe2x80x9cstackedxe2x80x9d face to face and secured to the burner head by a cap which surrounds them, which is screwed onto the burner head, and which has a shoulder bearing against the periphery of the nozzle-carrying spray piece.
An object of the present invention is to propose improved burners, in particular burners of the kind described in patent FR 2 766 557 (Pillard).
The invention seeks to simplify the implementation of liquid fuel atomizers, and in particular those described in patents FR 1 305 391 and FR 2 641 365, while leaving their functions unchanged and providing performance that is at least as good.
The invention seeks; to optimize the atomizer/burner xe2x80x9cpairxe2x80x9d by improvements relating specifically to the design of atomizers and dismountable liquid fuel atomizing heads, and also concerning the burner bodies proper, so as to make it easy to control optimization of this xe2x80x9cpairxe2x80x9d.
To this end, in a first aspect, the invention consists in providing a nozzle-carrier piece suitable for being removably mounted on an atomizer head of a liquid fuel burner, the piece including a rotary link member for constraining it to turn with said head, which member is preferably integrated in the structure of the nozzle-carrier piece to increase the strength of the link and avoid any faulty relative positioning, in particular during repeated disassembly and reassembly operations for maintenance purposes.
By means of this rotary constraint, the angle of orientation about the axis of the burner of the liquid fuel jets produced by the nozzle-carrier piece can be adjusted to correspond exactly to the angular position of the secondary air injection nozzles. Specifically with the burners shown in FIG. 2 of patents EP 0 774 620 and U.S. Pat. No. 5,562,437, this makes it possible to adjust and control the angular offset that exists between the liquid fuel injection orifice and the position of the secondary nozzle, which depends on,the direction of the xe2x80x9crotationxe2x80x9d imparted by the burner to the combustion air. The invention thus makes it possible to control the angular position of the piece in which the liquid fuel outlet orifices are made in such a manner that the angle of orientation of the liquid fuel injection head can be adjusted accurately on a case-by-case basis to optimize combustion.
In a preferred embodiment of the nozzle-carrier piece, it comprises a cylindrical wall of circular section (a hollow tube) which defines a cylindrical cavity, which is extended and/or partially closed at a first end by a thick transverse web (or wall) in which orifices are made to form the fuel spray nozzles, and which is open at its second end. This end is provided with a notch (or cutout) extending over a portion of its circumference, e.g. half its circumference. This end portion thus forms the rotary link member by engaging with little clearance in a cavity or depression of complementary shape formed at the end of the head which receives said removable nozzle-carrier piece.
Said cavity defined by the cylindrical body of the nozzle-carrier piece serves to receive the other pieces removably mounted on the head of the burner which serves to form a fuel jet, to mix it with the auxiliary fluid, and to transport and guide the fluid and the fuel to the nozzles.
As described in patent FR 2 641 365, the nozzle-carrier piece and the head can be releasably assembled by means of a threaded sleeve (or nut) which surrounds the nozzle-carrier piece while bearing against the periphery of its front face, which sleeve is screwed onto the threaded end of the burner head.
The end of the nozzle-carrier piece including the rotary link member preferably also includes a link member operating in longitudinal translation and suitable for co-operating with a complementary member provided at the end of the head. This disposition facilitates assembly and disassembly by enabling the nozzle-carrier piece and the removable pieces it contains to be hooked (and/or suspended) cantilevered out at the end of the head with the longitudinal axis common to the head and the nozzle-carrier piece being maintained substantially horizontal. This makes it possible to engage the sleeve around the nozzle-carrier piece without holding the piece, since it remains properly aligned with and pressed against the end of the head during this operation. The longitudinal link member is preferably integrated in the nozzle-carrier piece and is in the form of a projection such as a rib provided on the inside face of the non-cutout portion of said second end which extends over a portion of its circumference. The two integrated link members thus form a kind of hook suitable for engaging in a corresponding circumferential groove provided at the end of the head.
The advantages that result from the nozzle-carrier piece of the invention are further improved when, in a second aspect of the invention, a xe2x80x9ccompoundxe2x80x9d atomizer is used which is a single piece integrating nearly all of the functions normally provided by a plurality of pieces placed end to end (xe2x80x9cstackedxe2x80x9d).
Thus, in another aspect, the invention consists in providing a removable atomizer for a liquid fuel spraying burner head, which atomizer has a calibrated longitudinal central orifice for passing the fuel and a plurality of longitudinal peripheral orifices for passing an auxiliary spray fluid that open out at each end in respective annular grooves provided in the two faces of the atomizer, each of said faces presenting grooves that are substantially radial. On a xe2x80x9crearxe2x80x9d face, the one-piece atomizer also includes a depression surrounding said central orifice and into which tangential channels open out so that when the atomizer is mounted between the end of the head and a removable nozzle-carrier piece by a fixing member which is in the form of a sleeve, the depression co-operating with the end of the head to form an upstream chamber for swirling the fuel carried by these channels. Where appropriate, the atomizer can have a second depression suitable for co-operating with the nozzle-carrier piece to define an emulsion mixing chamber. The atomizer has radial or tangential channels on a second or xe2x80x9cfrontxe2x80x9d face that open out around the central orifice so as to enable the fuel jet leaving the central orifice to be mixed with the spray fluid.
In other words, the invention consists in proposing a single removable piece for a liquid fuel burner head, which piece has means (including tangential injection channels and a frustoconical hollow face) for guiding the flow of fuel so as to enable a conical jet to be formed at the outlet of a central orifice, means (longitudinal channels) for allowing an auxiliary fluid to pass through the piece, and means (radial or tangential channels) for guiding the fluid downstream from the central orifice and enabling the fluid to be put into contact with the jet of fuel.
Because of its small thickness, this piece is easily received inside a housing (bore) provided in the nozzle-carrier piece.